Heat exchangers are devices built for efficient heat transfer from one fluid to another. Conventional heat exchangers accomplish this heat transfer using a wide variety of interfaces and fluids. This invention is concerned with indirect heat transfer between two fluids of different temperatures across a dividing wall. More specifically, this invention is concerned with an indirect air-to-air heat exchanger, for use in high temperature applications, which uses an array of parallel tubes extending lengthwise within an elongate hollow vessel. The array of tubes is supported at each end of the vessel using a tube sheet. Tube sheets are used to receive the terminal ends of the tubes such that the tubes extend in a direction normal to the tube sheet face. The terminal ends of the tubes are seated within through channels in the tube sheet that allows fluid to pass between the interior of the tube and the opposing side of the tube sheet. The tubes and tube sheets are enclosed by a housing to form the heat exchanger vessel. In the ideal heat exchanger, there is no fluid leakage at the interface of the tube sheet and vessel walls, and there is no fluid leakage at the interface of the tube sheet and tube. The vessel housing typically includes a dome or some other form of enclosure at each end of the vessel which channels fluid to or from the tube sheet. The heat exchanger vessel is also provided with transversely aligned inlet and outlet ports which allow a second fluid to flow within the body of the vessel about the exterior of the tubes.
In such heat exchangers, a first fluid is passed from within a dome at a first end of the heat exchanger, through the tube sheet, through the interior of each tube within the tube array, through a second tube sheet, exiting through a second dome at the second end of the heat exchanger. A second fluid enters the body of the heat exchanger vessel through an inlet port such that it passes transversely through the tube array, passing about the exterior of the tubes, and exiting the vessel via the outlet port. The heat exchangers may be used as described as a single unit, or may be attached in series, dome to dome, with additional vessels to form a heat exchange system.
It is well understood that heat transfer is equally efficient regardless of whether the heating fluid is designated to be the first fluid and the heated fluid to be second fluid, as it is to allow the opposite to be the case. For purposes of discussion of this invention, we will consider the first fluid to be the fluid to be heated, and the second fluid to be the heating fluid.
Conventional heat exchangers, operating in the temperature range of 800 to 1400 degrees F., are constructed using metal tubes and tubes sheets. Typically, the metal tubes are secured to metal tube sheets by welding, or other well-known means. Such heat exchangers fail when operated at higher temperatures, and have a short life span when used with corrosive fluids as found in exhaust gases from industrial operations.
Heat exchangers that must operate in more severe conditions, as found in this invention, are fabricated with ceramic components. Such heat exchangers function well in moderate (1000 degrees F.) to high (2800 degrees F.) temperatures and are resistant to corrosive fluids. Ceramic tubes and tube sheets are well suited to use in severe operating conditions. However, the material properties of ceramics generate other design considerations. For example, loads need to be distributed evenly across the tube array to prevent any one tube from being overloaded. Thermal expansion of both the tubes and the tube sheets needs to be considered in the design so as to avoid additional stresses at the interface between these components. Finally, fluid leakage between the first and second fluids, such as found at the interface between tube and tube sheet, as well as between the tube sheet and vessel walls must be addressed.
The prior art ceramic tube sheets, such as the tube sheet disclosed in U.S. Pat. No. 5,979,543 to Graham, have been formed of plural individual ceramic tiles, each ceramic tile receiving and supporting multiple ceramic tube-ends. The individual ceramic tiles are then assembled and cemented together to form a generally planar tube sheet. Disadvantages to this type of tube sheet are fluid leakage at the cemented joint between tiles, and difficulty obtaining exact and precise alignment of tiles both within a tube sheet and between tube sheet pairs. Precise alignment between tube sheet pairs is required since it prevents problems with tube assembly, and insures that the tubes are equally loaded during operation.